1. Field of the Invention
The present invention relates to a fixing apparatus preferably for use in an electrophotographic image forming apparatus, and to an image forming apparatus having the same.
2. Description of the Related Art
In an image formation using an electrophotographic system, a photoreceptor charged with uniform electric potential is exposed to light in accordance with image information so that an electrostatic latent image is formed. The formed electrostatic latent image is developed by a developer so as to be visualized. The visualized image is transferred on a recording paper or the like, and the transferred developer on the recording paper is made to be fixed so as to form a solid recording image.
The fixing apparatus used for such image formation, is generally composed of a heating roller and a pressure roller, which are such configured that in passing the recording paper on which the developer for forming a visualized image through a pressure contact region (hereinafter referred to as a nip section) of the heating roller and the pressure roller which is formed by pressing the pressure roller against the heating roller, unfixed developer is fused and fixed by heating of the heating roller and pressing of the pressure roller.
During a fixing operation in the fixing apparatus, there sometimes occurs a so-called hot offset that the developer fused on the nip section of the both rollers is not all fixed on the recording paper, but a part of the developer is attached to a surface of the roller. For instance, the developer attached to the heating roller is transferred on a portion which should be properly a white base, on a recording paper on which the developer is to be subsequently fixed, with the result that an image defect is made to occur.
Moreover, on the pressure roller, the developer which has already fixed to a back surface of the conveyed recording paper, for instance as in a case of duplex print, is sometimes fused again by heat in passing through the nip section and a part of the developer is transferred and attached to the pressure roller. The developer thus attached to the pressure roller may cause the image defect and further, may cause a soil of the back surface of the recording paper.
The image defect caused by the hot offset in the fixing apparatus sometimes remains, in a case of black-and-white print, mere defects such as a fog in a white base of a formed image, a soil on the back surface of the recording paper, or the like in a tolerable range. However, in a case of full-color print, since a developer having a color different from a prescribed one is transferred from the both rollers, there often occur practically intolerable defects.
As a related art for solving such a problem, there is an apparatus having fixing roller cleaning means on the both rollers provided in a fixing apparatus (refer to Japanese Unexamined Patent Publication JP-A 2003-107952).
FIG. 12 is a schematic view showing a configuration of fixing roller cleaning means 1 provided in a related art fixing apparatus. FIG. 12 illustrates the fixing roller cleaning means 1 provided on a heating roller 2 in the fixing apparatus.
The fixing roller cleaning means 1 comprises a feeding roller 4 for feeding a belt-shaped cleaning member 3 which has been previously rolled up, a winding roller 5 for taking up the cleaning member 3 fed from the feeding roller 4, and a pressure-contact roller 6 (also referred to as a web pressure-contact roller) provided between the feeding roller 4 and the winding roller 5 so as to press the cleaning member 3 on the heating roller 2.
The fixing roller cleaning means 1 presses the cleaning member 3 on the heating roller 2 which is rotated in an arrow sign 7 direction in a state where the winding roller 5, the feeding roller 4, and the pressure-contact roller 6 are made to be at rest without being rotated so that the heating roller 2 and the cleaning member 3 are made to be slidingly scrubbed. By so doing, a developer 8a attached in a fused state to an outer circumferential surface of the heating roller 2 is removed and the removed developer 8b is accumulated, still in a substantially fused state, in a gap formed by the cleaning member 3 located between the pressure-contact roller 6 and the winding roller 5, and the surface of the heating roller 2.
When the developer 8b accumulated in the gap reaches a certain amount level, the fixing roller cleaning means 1 operates the winding roller 5 for take-up in an arrow sign 9 direction so as to take up the cleaning member 3 only by a predetermined length, with the result that the developer 8b is made to detach from the surface of the heating roller 2 in a state where the developer 8b is attached to the cleaning member 3.
The related art fixing roller cleaning means 1 can prevent the hot offset by cleaning the developer on the surface of the fixing roller. However, the following problems arise because the fixing roller cleaning means 1 is provided in a state of being constantly in contact with the fixing roller.
The fixing apparatus having the fixing roller cleaning means 1 is generally mounted on an electrophotographic image forming apparatus to be operated. When the image forming apparatus executes an image forming operation, the temperature of the fixing roller rises up to a predetermined fixing temperature. However, the image forming operation is not a nonstop and successive motion, but an intermittent operation. Accordingly, when the image forming operation is not in execution, the image forming apparatus becomes a state of low-power consumption, that is so-called energy-saving mode, or a shut off mode indicating a state where the apparatus itself is turned off. In the energy-saving mode, a temperature of the fixing roller of the fixing apparatus declines down to a relatively low temperature, that is a softening temperature of the developer or a temperature lower than the softening temperature. In the shut off mode, the temperature of the fixing roller of the fixing apparatus declines down to a room temperature which is an environmental temperature.
Consequently, when the image forming operation is not in execution, a temperature of the developer 8b on the fixing roller, which is captured by the cleaning member 3 also declines down to a temperature or a temperature lower than the softening temperature. When the temperature of the developer 8b thus declines down to a temperature lower than the softening temperature, the developer 8b becomes solidified. Among the developers, a black developer contains carbon and therefore, it becomes hard when solidified.
In a state where such a solidified and hard developer exists between the fixing roller and the cleaning member 3, for instance, a power source of the apparatus is turned on and the fixing roller starts to rotate until its temperature rises up to the predetermined fixing temperature. At this time, the surface of the fixing roller is slidingly scrubbed by the solidified developer, with the result that the surface of the fixing roller is damaged. When fixing is conducted by use of the fixing roller having a scratch on the surface thereof, there arises a problem that uneven fixing occurs on the formed image.